In recent years, a perforated soundproof structure for insulating sounds by the Helmholz resonance principle by oppositely arranging an internal plate having a number of through-holes formed on the whole surface and an external plate through an air layer have attracted attention. For example, in Japanese Patent Application Laid-open (Kokai) No. 6-298014, there is disclosed, focusing on that the general equation of the Helmholz resonance principle is “f=(c/2π)×√{square root over ( )}{β/(t+1.6b)d}”, a perforated sound insulating structure constituted to efficiently reduce noise of a specified resonance frequency f based on this general equation. In the above general equation, the resonance frequency f is represented by use of sound velocity c, open area ratio β, board thickness of internal plate t, hole diameter b and layer thickness of back air layer d as parameters.
In such a perforated soundproof structure constituted based on the general equation of the Helmholz resonance principle as in the past, the absorption coefficient to noises of frequencies other than the resonance frequency f can be extremely lowered depending on the way to combine the parameters. Therefore, it sometimes cannot exhibit sufficient sound absorbing performance to noises containing a plurality of frequencies as peak components.
Namely, the present inventors examined the relation between absorption coefficient α and frequency with parameters determined based on the above general equation to attain, for example, a resonance frequency f of 750 Hz. Consequently, as shown in FIG. 20, it was confirmed that some structures show sound absorbing characteristics that the peak value of the absorption coefficient α appears at 750 Hz that is the resonance frequency f, and the absorption coefficient α sharply drops from this peak value. In this case, when “0.3” is set as the threshold of the absorption coefficient α for exhibiting sufficient sound absorbing performance, the frequency bandwidth of sound absorbing characteristics in this threshold is about 41 Hz, which indicates that sufficient sound absorbing performance can be exhibited only at a bandwidth of 6% of the resonance frequency f of 750 Hz.
Accordingly, the conventional structures, as described above, have the problem that noises of a wide frequency bandwidth cannot be sufficiently insulated because the sound absorbing performance to noises other than the resonance frequency f is often extremely inferior. They also have the problem that experimental manufacture must be repeated until parameters for excellent sound insulating performance can be obtained in the determination of parameters based on the above-mentioned general equation.
On the other hand, a drive mechanism such as engine is not only a generating source of noise but also a generating source of mechanical vibration. At this time, even if designed according to the above general equation of the Helmholz resonance principle, the noise-proof cover is excited by the vibration of the drive mechanism, and the noise-proof cover itself, as a result, vibrates to generate noise. Accordingly, its soundproof performance is insufficient as a noise-proof cover for automobile that is mechanically excited, too.
The present invention has a principle object to provide a perforated soundproof structure capable of surely exhibiting sufficient sound absorbing performance and a method of manufacturing the same.